partial solution to run-time environment configuration :
- add bool OsdGLDrawContext::SupportsAdaptiveTessellation() method
- modify glViewer to use that instead of #ifdefs

Note : this is not the final word on this as OSD really needs a more comprehensive
system to provide run-time information about available features to the client code.

fixes #111
removing more #ifdef's related to adaptive tessellation ability

fixes #111